- Email: [email protected]
Effect of post design on resistance to fracture of endodontically treated teeth with complete crowns
LLOYD
38. Goering A, Mueninghoff tooth. Part I: concept
L. Management for restorative
of the endodontically treated designs. J PROSTHET DENT
1983;49:340-5.
39. Gutman J. Preparation of endodontically treated teeth to receive a post-core restoration. J PROS= DENT 1977;38:413-9. 40. Trabert K, Caputo A, Abou-Rass M. Tooth fracture-a comparison of endodontic and restorative treatments. J Endodont 1978;4:341-5. 41. As&f D, Aviv I, Himmel R. A rapid dowel core construction technique. J PROSTHET DENT 1989;61:16-7. 42. Haddix JE, Mattison GD, Shulman CA, Pink FE. Post preparation techniques and their effect on the apical seal. J PRO~THET DENT 1990;64:515-9.
43. Bergmam B, Lundquist P, Sjogren U, Sundquist G. Restorative and endodontic results after treatment with cast post and cores. J PROSTI-IIW DENT 1989;61:10-25. 44. Zarb G, Bergman B, Clayton J, MacKay H. Prosthodontic treatment for par&By edentulous patients. St Louis: CV Mosby, 1978130-9. 45. Brandal J, Nicholls J, Harrington G. A comparison of three restorative techniques for endodontically treated anterior teeth. J PROSTHEX DENT 1987;58:161-5. 46. Sheets C. Dowel and core foundations. J PROWHET DENT 1970;23:5865.
47. Snyder D. Dowel-inlay repair of a loose crown of an endodontically volved tooth. Quint Int 1986;17:185-7.
in-
AND
PALIK
48. Johnson J, Schwartz N, Blackwell R. Evaluation and restoration of endodontically treated posterior teeth. J Am Dent Assoc 197693597~605. 49. Tilk M, Lommel T, Gerstein H. A study of mandibular and maxillary root widths to determine dowel size. J Endodont 1979$x79-82. 50. Hunter A, Feiglin B, Williams J. Effects of post placement on endodontically treated teeth. J PROST~ DENT 1989,62:166-172. 51. Caputo A, Standlee J. Pins and posts-Why, when and how. Dent Clin North Am 1976;20:299-311. 52. Barkhordar R, Radke R, Abbasi J. Effect of metal collars on resistance of endodontically treated teeth to root fracture. J PROSTHET DENT 1989;61:676-8.
53. Gerstein H, Evanson L. Precision posts or dowels. Illinois Dent J 1963;32:70-3. 54. Marshak B, Helft H, Filo R. Factors mitigating against the use of dowels in endodontically treated teeth. Quint Int 1988;19:417-21. 55. Colman H. Restoration of endodontically treated teeth. Dent Clin North Am 1979;23:647-62. Reprint requests to: DR. PATRICK M. LLOVD hfARQUElTI3 UNIVERWlY SCHOOL OF DEN~~TRY 694 NORTH SIXTEENTH MILWAUKEE, WI 53233
STREET
Effect of post design on resistance to fracture of endodontically treated teeth with complete crowns David Assif, DMD,a Avraham and Eyal Oren, DMDb
Bitenski,
MD, DMD,b
Raphael
Pilo, DMD,b
The Maurice and Gabriela Goldachleger School of Dental Medicine, Tel Aviv University, Tel Aviv, Israel The effect of post design on the fracture resistance of endodontically treated premolar6 restored with cast crowns was examined in vitro. The experimental model used cast posts and cores to test the eifect of post design in a post-core system with identical rigidity. Samples loaded on an Instron testing machine until failure revealed that post design did not influence the fracture resistance of endodontically treated premolar8 restored with complete cast crowns. There was also no statistically significant difference between restored teeth with or without cast posts and cores. (JPROSTHETDENT 1003;89:36-40.)
A
post-core system is usually inserted to restore damaged teeth after endodontic treatment.‘1 2 The functions of a dowel placed in the root canal of the restored tooth are (1) to disperse occlusalforces along the root; and (2) to provide retention of the core substituting for coronal tooth structure and supporting or retaining the final restoration.lm5 Techniques available for constructing a dowel and core are (1) the conventional dowel and core cast asa singleunit with the shapeof the dowel conforming to the morphology and Deputy Head, Department of Proathodontica; Clinical Senior Lecturer, Proethodontic Rehabilitation. bInetructor, Department of Proatodontics. 10/l/41126 %aordinator
36 0022-3913/93/$1.00
+ JO
of the root canal;i” (2) a dowel-core, prefabricated commercial post of various designsusedwith coresof various materials (such as amalgam,compositeresin, or glassionomer cement);l*3,5l6 and (3) cast dowel and core in which the post is identical to a prefabricated commercialpost. In the third technique, the commercialprefabricated plastic model of the post is usedas a pattern to cast the restoration.il 4 Controversy exists concerning dowel design for endodontically treated teeth. Some investigators claim that a cylindrical post dispersesthe occlusalforces evenly along the root,‘>’ whereasothers contend that a cylindrical dowel causesstress concentration in the apical surface of the ro0t.s gSeveralresearchersalsodemonstratedthat tapered or tapered-end dowels caused a wedging effect,‘plO but
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others suggested that tapered posts distributed the stress evenly along the ro0t.s The differences between the reported studies may originate in variations of post-core rigidity. Several studies were conducted to examine posts and cores of various metals and materials, with different rigidity.6, 11-14When a system with components of different rigidity was loaded, the more rigid component was capable of resisting greater forces without distortion. Stress was transferred to the less rigid component as loading continues, causing the weaker component to fail and relieving the stress.15 In endodontically treated teeth, strong metal posts cause stress concentration in the less rigid root, resulting in root fracture. Another explanation may be that the post-core is restored with a complete cast crown. Studies related to the strength and resistance of posts to compressive forces on the post structure have been conducted, but these investigations have not been tested clinically.16~ l7 A tooth with a post is commonly restored with an artificial crown having the finish line on healthy tooth structure. Several studies have reported no significant difference in the compressive strength of teeth with different types of posts, provided the teeth were restored with a complete crown and gingival margins within sound tooth structure.5p 13,l4 This present study examined the effect of post design on the fracture resistance of endodontically treated teeth under simulated occlusal loads. A post-core system of identical rigidity was restored with a complete cast crown having 2 mm margins on healthy tooth structure apical to the border of the core.
MATERIAL
AND
METHODS
Forty-one freshly extracted, intact single-rooted premolam of similar dimensions were selected for the investigation and preserved in physiologic saline after endodontic therapy. The canals were successively reamed with K files through No. 70 and obturated with gutta-percha by use of alateral condensation technique with AH-26 (De-Tray AG, Zurich, Switzerland) cementing medium. Teeth were randomly divided in three groups of 10 teeth, and one group of 11 teeth. In three of the groups, the anatomic crown was sectioned vertically to the long axis, 2 mm coronal to the cementoenamel junction (CXJ), with a water spray-cooled diamond at high speed. Gutta-percha was removed from the canal to a depth of 9 mm and widened 1 mm in diameter with a heat plugger for the posts. Group 1. Conventional cast dowel-core (11 teeth). The canal was prepared with a Pessoreamer (Maillefer Ballaigues,Switzerland). An impressionof the canal wasmade with Duralay (Reliance Dental Mfg. Co., Worth, Ill.) selfcuring acrylic resin, and the core was completed with the same acrylic resin to 3 mm coronal of the sectioned anatomic crown. The acrylic pattern wascast in Palorag 33 type 3 nonprecioushigh strength alloy (Cendres& Me’taux SA, Biel-Biene, Switzerland).
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Group 2. Cast cylindrical dowel and core. The canalwas prepared for a parallel-sided cylindrical post by use of matching drills (Para-Post drills, Whaledent, Inc., New York, N. Y.). A matching cylindrical laboratory burnout casting post for indirect technique (Whaledent, Inc.) wasadapted to the prepared canal and the core was formed with Duralay acrylic resin to 3 :mmcoronal to the sectionedsurfaceof the anatomic crown. The plastic pattern was then cast in a nonprecious alloy (Pallorag 33). Group 3. Cast cylindrical tapered-end dowel and core. The canal was prepared for the post witch a tapered-end Para-Post drill. A replica of the stainlesssteel post was madeto fabricate a tapered-end plastic burnout post with a vinyl polysiloxane impression (ExaAex, GC Dental Industrial Corp, Tokyo, Japan). A self-curing acrylic resin (Duralay) was injected in the impressions.The resulting acrylic posts were positioned in the canals, the core was completed with Duralay acrylic resinto 3 mmcoronal of the cut-tooth surface, and the patterns were cast,in a nonprecious alloy (Pallorag 33). Group 4. Gutta-percha wasremoved to a depth of 9 mm from the canal with hot plugger, and the endodontic access cavity was closed with Ketac-fill (ESPE GmBH & Co., Seefeld, Germany) glassionomer cement. In the three experimental groups, cast dowel-coreswere cemented with zinc-phosphate cement (Harvard cement, Richter and Hoffman, Berlin, Germany), according to the manufacturer’s instructions. Copal varnish wasapplied t.o the cementation surface and the teeth were returned to physiologic saline. Seven days after cementation, all of the teeth, including the control group, were prepared for a complete cast crown with a chamfer finish line at the CEJ. The finish line was 2 mm apical to the border of the core previously described in dowel and calrepreparation. ImpressionsaIfthe teeth were madewith vinyl polysiloxane, and die stone (Silky, Rock-Whip Mix Co., Louisville, Ky.) was used to cast the impressionsaccording to the manufacturer’s instructions. Complete veneer cast crowns were constructed with a 20 pm die spacer used approximately 1 mm from the finish line, cast in Pallorag 33 nonpreciousalloy (Cendres& Me’taux SA) and cementedwith zinc phosphate cement (Harvard) according to the manufacturer’s instructions. Each tooth was embeddedin a self-curing acrylic resin cylinder sothat the long axis of the tooth wasparallel to the cylinder, and the acrylic resin covered the root to 2 mm apical to the CEJ. The sampleswere securedin an Instron testing machine (Instron 1026, Instron Co., Canton, Mass.). A continuous increasingcompressiveforce wasapplied to the facial CUSP in the axio-occlusalline angle 30 degreesto the long axis of the tooth at a crossheadspeedof 2 mm/minute until failure. The magnitude of the force causing failure and the shapeof the fracture were recorded. Statistical analysisof
37
ASSIF
Table
I. Failure loads (kg) in the various groups (in increasingorder) Group
Sample
Cast
No.
1 2 3 4 5 6 7 8 9 10 11
c3 Tapered-end post cast dowel-core
c4 Control
92.5 140 187.5
170 185 232.5
210 245 250 250
242.5 260 275 310
270 320 395 440
315 342.5 435 -
137.5 175 195 202.5 205 207.5 210 255 260 435 -
125 162.5 185 210 220 230 245 250 300 325 -
Group
Mean Median SD
No. and features
c2 Cylindrical post cast dowel-core
Cl dowel-core
Table II. Mean, median, and standard deviation for each group No.
Cl
c2
c3
c4
254.2 250.0 102.3
276.1 267.5 78.3
228.2 206.2 80.7
225.2 225.0 60.1
the data was computed to reveal significant correlations between the groups.
RESULTS The design of the dowel did not influence the fracture resistanceof endodontically treated teeth when the dowel and core had identical rigidity and the tooth was restored with a complete cast crown with margins placed 2 mm on sound tooth structure. Tables I and II list the forces that causedfailure in the various groups. The meansand standard deviations were not calculated becausea wide range of compressiveforces was recorded for each group. Consequently, the MannWhitney aparametic test wasusedto determine the design that was statistically significant. The results revealed no statistically significant difference between groups (Table III). This indicated that the dowel designdid not effect the fracture resistanceof endodontically treated teeth if the tooth was restored with a complete cast crown. Mode of failure. The mode of failure in all of the teeth was oblique root fracture. There were no vertical or longitudinal fractures paralleling the long axis of the teeth (Fig. l), and no cohesive or adhesive failure of the cement. In group 2 with a cylindrical post, there was one dowel fracture. Two samplesin group 1 and one samplein group 3 displayed a bent post tip.
38
ET AL
DISCUSSION In previous studies of the effect of dowel designon the fracture resistanceof endodontically treated teeth, forces were applied directly to the post or core.11l12,16-lgForces were alsoapplied directly to the post or core in photoelastic studies for analysis of stress distribution of different post designs77 lo*20,21This situation has no clinical parallel, becausein most patients the core was restored with a complete crown having a 2 mm margin on healthy tooth structure. The rigidity of the various componentsin the research of dowel-core systems has been an additional problem.* Studies on dowel and cores with different rigidity revealed that numerousfailures were causedby a fractured core, whereasfailures of cast dowel-coresystemshave been characterized by fractured roots69u-13,l7 The experimental model in this study was a cast post. core systemwith completecrowns,with identical alloy used for all castings.This isolation enabledtesting of the influenceof post designon the fracture resistanceof endodontically treated teeth in a standardized reconstructive system. Photoelastic studiesrelating to stressresulting from different post configurations confirmed apical stressconcentration in cylindrical dowels, whereas tapered dowels causedstressconcentration or a wedging effect in the apex and the CEJ.7$8,l”p20Cores covered by a complete cast crown with a 2 mm margin on healthy tooth structure exhibited reduced stressconcentration at the apex and stress concentrated in the CEJ despite the presenceof the dowel or its designs The influence of core coverageby a complete cast crown with margins on healthy tooth structure wasexamined in similar studies evaluating the fracture resistanceof endo-
*References 5, 6,
11, 12, 17, 20, 22.
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Cl
63.6%
c2
36.4% Fig.
1.
Fracture patterns in the various groups.
dontically treated teeth. There were significant differences between various dowel configurations and different core materials without a complete crown. Differences became insignificant after core coverage by a complete crown57 13*14*16*22,23 Conversely, differences were discovered between dowel design and core materials with teeth restored with complete cast crowns,6-12 and in one study, cast crowns were fabricated but not cemented.12 This study supported the assumption that post design did not influence the resistance to fracture of endodontitally treated teeth, provided the core was covered by a complete cast crown with a 2 mm margin on healthy tooth structure. The crown changed the distribution of forces to the root and the post and core complex, rendering the post characteristics insignificant. The forces responsible for failure were considerably higher than the maximal physiologic forces acting on the teeth in the oral cavity (Table I). A wide range in the average forces were discovered with in vitro studies of dowel-core systems restored with complete cast crowns.5’ 6*12-14*22*24However, there was some similarity between the magnitude of the forces causing failure in this research and in related studies.14p l6 Several factors contribute to the fracture resistance of teeth: (1) classification of the teeth (incisors versus molars); (2) degree of calcification; (3) distance from the CEJ at which loads are applied; (4) direction of force; (5) core height coronal to prepared tooth; and (6) the ferruling effect of the artificial crown margins. The rigidity of the material for embedding the tooth can also have an influence. A conservative approach of 1 mm in diameter was used in preparation of the root canal to preserve tooth structure. Preserving tooth structure during canal preparation is crucial to prevent stress concentration at the CEJ of the tooth21 and provide resistance to tooth fracture.g, 25 The dowel configuration is essential in a root if a thin dentinal wall remains after root canal preparation. This assumption
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DENTISTRY
Table
III.
Comparison of all possible group couples
Groups compared
P
Cl-C2 Cl-C3 Cl-C4 C2-C3 C2-C4 c3-c4
0.65 0.40 0.46 0.12 0.13 0.70
Statistical signi6cance
--_
--
NS NS NS NS NS NS
was not examined in this study, but is an additional factor that could explain the difference in results between this study and researchthat examined dowelsof larger diameter. Therefore, the remaining dentinal thicknesswassmaller 6,24-26
It was impossibleto compare the forces causingfailure in this study with similar investigation becauseof numerous variables. Standard deviations of 20% to 40% were observedin the median values (Table II) asa result of wide distribution in all groups (Table I). This could be attributed to (1) composition and thickness of the cementing agent, which was not standardized in all specimens;(2) dissimilar posit:ionsof the dowel in the prepared site; (3) variations in tooth dimensions;(4) minute variations in the morphology of root canals; and (5) variations in dentin causedby different water content, pulpal conditions before tooth extraction, patient’s age, and direction of dentinal channels. Similar standard deviations have been documented in the literature.16-1gx 23,24 The fracture lines in teeth containing a dowel-core USUally passedthrough the apex, but in some instances the fracture line passedthrough the apical third of the post (Fig. 1). The fracture line in the control group wassimil.ar in angle to other groups, but more coronal, and in the lingual surface or the surfaceopposite t,o the applied force, it 39
ASSIF
passed at the point where the tooth emerged from the acrylic resin cylinder. The position and angle of this fracture line were caused by the fulcrum in the lingual surface between the tooth embedded in the acrylic resin and the tooth surface coronal to the cylinder. The tooth then acted as a type 1 lever arm during force application on the facial surface. The fracture line passed more apically in the experimental groups containing a dowel; thus the dowel caused stress distribution along its length and shifted the stress concentration apically. In this study, there were no fractures parallel to the long axis of the root as in similar investigations when forces were applied directly to the core.23*24 Photoelastic studies disclosed that when forces were applied directly on tapered dowels, there was a wedging effect on the root?,sy lo However, if the core was covered by a complete crown, the wedging effect disappeared with a shift of stress concentration to the CElJ.8 This explained the longitudinal fractures in teeth with a dowel-core system without an artificial crown, but was not evident in teeth restored with a complete crown having 2 mm margins on healthy tooth structure. CONCLUSIONS The post design did not influence the fracture resistance of endodontically treated teeth having a dowel-core system of identical rigidity and restoration with a complete cast crown having a 2 mm margin on healthy tooth structure. The selection of a dowel should be based on a system that preserves the most tooth structure and possesses suitable retention of the core for restoration of the tooth. If the anatomic crown is sufficiently preserved and core retention can be achieved from within the natural crown, or if completion of the coronal surface is unnecessary, a dowel is not required. We express our deep appreciation to Dr. Tamar Brosh PhD (Engineering), Department of Oral Rehabilitation, for her invaluable assistance in performing the experiment; to Mrs. Ilana Gelerenter, Statistics Laboratory, for analysis of the data; to Whaledent International and their representatives in Israel for their assistance in acquiring equipment for the experiment; to Dentor Laboratories, Tel Aviv, for the casting of the post-cores and crowns; and to Ms. Rita Laxar for editorial assistance. REFERENCES Trabert KC, Coony JF’. The endodontically treated tooth. Restorative concepts and techniques. Dent Clin North Am 1984;28:923-51. 2. Colman HL. Restoration of endodonticslly treated teeth. Dent Clin 1.
North Am 1979;23:647-62. 3. Sokol DJ. Effective use of current DENT 1984,52:231-4.
40
ET AL
4. Baraban DJ. The restoration of endodontically treated teeth: an update. J PROSTHET DENT 198&59:553-S. 5. Hoag EP, Dwyer TG. A comparative evaluation of three post and core techniques. J PROSTHET DENT 1982;47:177-81. 6. Brandal JL, Nicholls JI, Harrington GW. A comparison of three restorative techniques for endodontically treated anterior teeth. J PROSTHET DENT
1987;58:161-5.
7. Coony JP, Caputo AA, Trabert KC. Retention and stress distribution of tapered-end endodontic posts. J PROSTHET DENT 1986;55:540-6. 8. Assif D, Oren E, Marshak BL, Aviv I. Photoelastic analysis of stress transfer by endodontically treated teeth to the supporting structure using different restorative techniques. J PROSTHET DENT 1989;61:535-43. 9. Standlee JP, Caputo AA, Holcomb J, Trabert KC. The retentive and stress distributing properties of a threaded endodontic dowel. J PROSTHET DENT
1980;44:398-404.
10. Standlee JP, Caputo AA, Pollack MM. Analysis of stress distribution by endodontic posts. Oral Surg, Oral Med, Oral Path01 1972;33:952-60. 11. Kantor ME, Pines MS. A comparative study of restorative techniques for pulpless teeth. J PROSTHET DENT 1977;38:405-12. 12. Chau RW, Bryant RW. Post-core foundations for endodontically treated teeth. J PROSTHET DENT 1982;48:401-6. 13. Gelfand M, Goldman M, Sunderman EJ. Effect of complete veneer crowns on the compressive strength of endodontically treated posterior teeth. J PROSTHET DENT 1984;52:635-8. 14. Nayyar A, McDonald TR, Turner F, Koth DL. Strength of premolar corona-radicular buildups restored with cast crowns [Abstract]. J Dent Res 1982;61:70. 15. Caputo AA, Standlee JP. Biomechanics in clinical dentistry. 1st ed. Chicago: Quintessence Publishing Co, 1987:134. 16. Michelich R, Nayyar A, Leonard L. Mechanical properties of amalgam core-buildups for endodontically treated premolars [Abstract]. J Dent Res 1981;60:1283. 17. Greenfeld RS, Roydhouse RH, Marshall FJ, Schoner B. A comparison of two post systems under applied compressive-shear loads. J PROSTHET DENT
1989;61:17-24.
18. Gusy GE, Nicholls JI. In vitro comparison of intact endodontically treated teeth with and without endo-post reinforcement. J PROSTHET DENT
1979;42:39-44.
19. Trope M, Malts DO, Tronstad L. Resistance to fracture of restored endodontically treated teeth. Endodon Dent Traumatol 1985;1:108-11. 20. Burns DA, Krause WR, Douglas HB, Burns DR. Stress distribution surrounding endodontic posts. J PROSTHET DENT 1990;64:412-8. 21. Hunter AJ, Feiglin B, Williams JF. Effects of post placement on endodontically treated teeth. J PROSTHET DENT 1989;62:166-72. 22. Volwiler RA, Nicholls JI, Harrington GW. A comparison of three core buildup materials used in conjunction with two post systems in endodontically treated anterior teeth. J Endodont 1989;15:355-61. 23. Barkhorder RA, Radke R, Abbasi J. Effect of metal collars on resistance of endodontically treated teeth to root fracture. J PROSTHET DENT 198%61:676-S.
24. Sorensen JA, Engelman MJ. Effect of post design on fracture resistance of endodontically treated teeth. J PROSTHET DENT 1990$X419-24. 25. Tjian AI-IL, Whaug SB. Resistance to root fracture of dowel channels with various thicknesses of buccal dentin walls. J PROSTHJZT DENT 1985;53:496-500.
26. Sorensen JA, Engelman MJ. Ferrule design and fracture resistance of endodontically treated teeth. J PROSTHET DENT 1990,63:529-36. Reprint
requests
to:
DR. DAVID ASSIF SECTION OF ORAL REHABILITATION THE MAURICE AND GABRIELA GOLDSCHLECER TEL Av~v UNIVEFWTY TEL AVIV 69978 ISRAEL
SCHOOL OF DENTAL
MEDICINE
core and post concepts. J PROSTHET
JANUARY
1993
VOLUME
69
NUMBER
1
38. Goering A, Mueninghoff tooth. Part I: concept
L. Management for restorative
of the endodontically treated designs. J PROSTHET DENT
1983;49:340-5.
39. Gutman J. Preparation of endodontically treated teeth to receive a post-core restoration. J PROS= DENT 1977;38:413-9. 40. Trabert K, Caputo A, Abou-Rass M. Tooth fracture-a comparison of endodontic and restorative treatments. J Endodont 1978;4:341-5. 41. As&f D, Aviv I, Himmel R. A rapid dowel core construction technique. J PROSTHET DENT 1989;61:16-7. 42. Haddix JE, Mattison GD, Shulman CA, Pink FE. Post preparation techniques and their effect on the apical seal. J PRO~THET DENT 1990;64:515-9.
43. Bergmam B, Lundquist P, Sjogren U, Sundquist G. Restorative and endodontic results after treatment with cast post and cores. J PROSTI-IIW DENT 1989;61:10-25. 44. Zarb G, Bergman B, Clayton J, MacKay H. Prosthodontic treatment for par&By edentulous patients. St Louis: CV Mosby, 1978130-9. 45. Brandal J, Nicholls J, Harrington G. A comparison of three restorative techniques for endodontically treated anterior teeth. J PROSTHEX DENT 1987;58:161-5. 46. Sheets C. Dowel and core foundations. J PROWHET DENT 1970;23:5865.
47. Snyder D. Dowel-inlay repair of a loose crown of an endodontically volved tooth. Quint Int 1986;17:185-7.
in-
AND
PALIK
48. Johnson J, Schwartz N, Blackwell R. Evaluation and restoration of endodontically treated posterior teeth. J Am Dent Assoc 197693597~605. 49. Tilk M, Lommel T, Gerstein H. A study of mandibular and maxillary root widths to determine dowel size. J Endodont 1979$x79-82. 50. Hunter A, Feiglin B, Williams J. Effects of post placement on endodontically treated teeth. J PROST~ DENT 1989,62:166-172. 51. Caputo A, Standlee J. Pins and posts-Why, when and how. Dent Clin North Am 1976;20:299-311. 52. Barkhordar R, Radke R, Abbasi J. Effect of metal collars on resistance of endodontically treated teeth to root fracture. J PROSTHET DENT 1989;61:676-8.
53. Gerstein H, Evanson L. Precision posts or dowels. Illinois Dent J 1963;32:70-3. 54. Marshak B, Helft H, Filo R. Factors mitigating against the use of dowels in endodontically treated teeth. Quint Int 1988;19:417-21. 55. Colman H. Restoration of endodontically treated teeth. Dent Clin North Am 1979;23:647-62. Reprint requests to: DR. PATRICK M. LLOVD hfARQUElTI3 UNIVERWlY SCHOOL OF DEN~~TRY 694 NORTH SIXTEENTH MILWAUKEE, WI 53233
STREET
Effect of post design on resistance to fracture of endodontically treated teeth with complete crowns David Assif, DMD,a Avraham and Eyal Oren, DMDb
Bitenski,
MD, DMD,b
Raphael
Pilo, DMD,b
The Maurice and Gabriela Goldachleger School of Dental Medicine, Tel Aviv University, Tel Aviv, Israel The effect of post design on the fracture resistance of endodontically treated premolar6 restored with cast crowns was examined in vitro. The experimental model used cast posts and cores to test the eifect of post design in a post-core system with identical rigidity. Samples loaded on an Instron testing machine until failure revealed that post design did not influence the fracture resistance of endodontically treated premolar8 restored with complete cast crowns. There was also no statistically significant difference between restored teeth with or without cast posts and cores. (JPROSTHETDENT 1003;89:36-40.)
A
post-core system is usually inserted to restore damaged teeth after endodontic treatment.‘1 2 The functions of a dowel placed in the root canal of the restored tooth are (1) to disperse occlusalforces along the root; and (2) to provide retention of the core substituting for coronal tooth structure and supporting or retaining the final restoration.lm5 Techniques available for constructing a dowel and core are (1) the conventional dowel and core cast asa singleunit with the shapeof the dowel conforming to the morphology and Deputy Head, Department of Proathodontica; Clinical Senior Lecturer, Proethodontic Rehabilitation. bInetructor, Department of Proatodontics. 10/l/41126 %aordinator
36 0022-3913/93/$1.00
+ JO
of the root canal;i” (2) a dowel-core, prefabricated commercial post of various designsusedwith coresof various materials (such as amalgam,compositeresin, or glassionomer cement);l*3,5l6 and (3) cast dowel and core in which the post is identical to a prefabricated commercialpost. In the third technique, the commercialprefabricated plastic model of the post is usedas a pattern to cast the restoration.il 4 Controversy exists concerning dowel design for endodontically treated teeth. Some investigators claim that a cylindrical post dispersesthe occlusalforces evenly along the root,‘>’ whereasothers contend that a cylindrical dowel causesstress concentration in the apical surface of the ro0t.s gSeveralresearchersalsodemonstratedthat tapered or tapered-end dowels caused a wedging effect,‘plO but
JANUARY
1883
VOLUME
69
NUMBER
1
POST
DESIGN
AND
RESISTANCE
T O FRACTURE
others suggested that tapered posts distributed the stress evenly along the ro0t.s The differences between the reported studies may originate in variations of post-core rigidity. Several studies were conducted to examine posts and cores of various metals and materials, with different rigidity.6, 11-14When a system with components of different rigidity was loaded, the more rigid component was capable of resisting greater forces without distortion. Stress was transferred to the less rigid component as loading continues, causing the weaker component to fail and relieving the stress.15 In endodontically treated teeth, strong metal posts cause stress concentration in the less rigid root, resulting in root fracture. Another explanation may be that the post-core is restored with a complete cast crown. Studies related to the strength and resistance of posts to compressive forces on the post structure have been conducted, but these investigations have not been tested clinically.16~ l7 A tooth with a post is commonly restored with an artificial crown having the finish line on healthy tooth structure. Several studies have reported no significant difference in the compressive strength of teeth with different types of posts, provided the teeth were restored with a complete crown and gingival margins within sound tooth structure.5p 13,l4 This present study examined the effect of post design on the fracture resistance of endodontically treated teeth under simulated occlusal loads. A post-core system of identical rigidity was restored with a complete cast crown having 2 mm margins on healthy tooth structure apical to the border of the core.
MATERIAL
AND
METHODS
Forty-one freshly extracted, intact single-rooted premolam of similar dimensions were selected for the investigation and preserved in physiologic saline after endodontic therapy. The canals were successively reamed with K files through No. 70 and obturated with gutta-percha by use of alateral condensation technique with AH-26 (De-Tray AG, Zurich, Switzerland) cementing medium. Teeth were randomly divided in three groups of 10 teeth, and one group of 11 teeth. In three of the groups, the anatomic crown was sectioned vertically to the long axis, 2 mm coronal to the cementoenamel junction (CXJ), with a water spray-cooled diamond at high speed. Gutta-percha was removed from the canal to a depth of 9 mm and widened 1 mm in diameter with a heat plugger for the posts. Group 1. Conventional cast dowel-core (11 teeth). The canal was prepared with a Pessoreamer (Maillefer Ballaigues,Switzerland). An impressionof the canal wasmade with Duralay (Reliance Dental Mfg. Co., Worth, Ill.) selfcuring acrylic resin, and the core was completed with the same acrylic resin to 3 mm coronal of the sectioned anatomic crown. The acrylic pattern wascast in Palorag 33 type 3 nonprecioushigh strength alloy (Cendres& Me’taux SA, Biel-Biene, Switzerland).
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Group 2. Cast cylindrical dowel and core. The canalwas prepared for a parallel-sided cylindrical post by use of matching drills (Para-Post drills, Whaledent, Inc., New York, N. Y.). A matching cylindrical laboratory burnout casting post for indirect technique (Whaledent, Inc.) wasadapted to the prepared canal and the core was formed with Duralay acrylic resin to 3 :mmcoronal to the sectionedsurfaceof the anatomic crown. The plastic pattern was then cast in a nonprecious alloy (Pallorag 33). Group 3. Cast cylindrical tapered-end dowel and core. The canal was prepared for the post witch a tapered-end Para-Post drill. A replica of the stainlesssteel post was madeto fabricate a tapered-end plastic burnout post with a vinyl polysiloxane impression (ExaAex, GC Dental Industrial Corp, Tokyo, Japan). A self-curing acrylic resin (Duralay) was injected in the impressions.The resulting acrylic posts were positioned in the canals, the core was completed with Duralay acrylic resinto 3 mmcoronal of the cut-tooth surface, and the patterns were cast,in a nonprecious alloy (Pallorag 33). Group 4. Gutta-percha wasremoved to a depth of 9 mm from the canal with hot plugger, and the endodontic access cavity was closed with Ketac-fill (ESPE GmBH & Co., Seefeld, Germany) glassionomer cement. In the three experimental groups, cast dowel-coreswere cemented with zinc-phosphate cement (Harvard cement, Richter and Hoffman, Berlin, Germany), according to the manufacturer’s instructions. Copal varnish wasapplied t.o the cementation surface and the teeth were returned to physiologic saline. Seven days after cementation, all of the teeth, including the control group, were prepared for a complete cast crown with a chamfer finish line at the CEJ. The finish line was 2 mm apical to the border of the core previously described in dowel and calrepreparation. ImpressionsaIfthe teeth were madewith vinyl polysiloxane, and die stone (Silky, Rock-Whip Mix Co., Louisville, Ky.) was used to cast the impressionsaccording to the manufacturer’s instructions. Complete veneer cast crowns were constructed with a 20 pm die spacer used approximately 1 mm from the finish line, cast in Pallorag 33 nonpreciousalloy (Cendres& Me’taux SA) and cementedwith zinc phosphate cement (Harvard) according to the manufacturer’s instructions. Each tooth was embeddedin a self-curing acrylic resin cylinder sothat the long axis of the tooth wasparallel to the cylinder, and the acrylic resin covered the root to 2 mm apical to the CEJ. The sampleswere securedin an Instron testing machine (Instron 1026, Instron Co., Canton, Mass.). A continuous increasingcompressiveforce wasapplied to the facial CUSP in the axio-occlusalline angle 30 degreesto the long axis of the tooth at a crossheadspeedof 2 mm/minute until failure. The magnitude of the force causing failure and the shapeof the fracture were recorded. Statistical analysisof
37
ASSIF
Table
I. Failure loads (kg) in the various groups (in increasingorder) Group
Sample
Cast
No.
1 2 3 4 5 6 7 8 9 10 11
c3 Tapered-end post cast dowel-core
c4 Control
92.5 140 187.5
170 185 232.5
210 245 250 250
242.5 260 275 310
270 320 395 440
315 342.5 435 -
137.5 175 195 202.5 205 207.5 210 255 260 435 -
125 162.5 185 210 220 230 245 250 300 325 -
Group
Mean Median SD
No. and features
c2 Cylindrical post cast dowel-core
Cl dowel-core
Table II. Mean, median, and standard deviation for each group No.
Cl
c2
c3
c4
254.2 250.0 102.3
276.1 267.5 78.3
228.2 206.2 80.7
225.2 225.0 60.1
the data was computed to reveal significant correlations between the groups.
RESULTS The design of the dowel did not influence the fracture resistanceof endodontically treated teeth when the dowel and core had identical rigidity and the tooth was restored with a complete cast crown with margins placed 2 mm on sound tooth structure. Tables I and II list the forces that causedfailure in the various groups. The meansand standard deviations were not calculated becausea wide range of compressiveforces was recorded for each group. Consequently, the MannWhitney aparametic test wasusedto determine the design that was statistically significant. The results revealed no statistically significant difference between groups (Table III). This indicated that the dowel designdid not effect the fracture resistanceof endodontically treated teeth if the tooth was restored with a complete cast crown. Mode of failure. The mode of failure in all of the teeth was oblique root fracture. There were no vertical or longitudinal fractures paralleling the long axis of the teeth (Fig. l), and no cohesive or adhesive failure of the cement. In group 2 with a cylindrical post, there was one dowel fracture. Two samplesin group 1 and one samplein group 3 displayed a bent post tip.
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DISCUSSION In previous studies of the effect of dowel designon the fracture resistanceof endodontically treated teeth, forces were applied directly to the post or core.11l12,16-lgForces were alsoapplied directly to the post or core in photoelastic studies for analysis of stress distribution of different post designs77 lo*20,21This situation has no clinical parallel, becausein most patients the core was restored with a complete crown having a 2 mm margin on healthy tooth structure. The rigidity of the various componentsin the research of dowel-core systems has been an additional problem.* Studies on dowel and cores with different rigidity revealed that numerousfailures were causedby a fractured core, whereasfailures of cast dowel-coresystemshave been characterized by fractured roots69u-13,l7 The experimental model in this study was a cast post. core systemwith completecrowns,with identical alloy used for all castings.This isolation enabledtesting of the influenceof post designon the fracture resistanceof endodontically treated teeth in a standardized reconstructive system. Photoelastic studiesrelating to stressresulting from different post configurations confirmed apical stressconcentration in cylindrical dowels, whereas tapered dowels causedstressconcentration or a wedging effect in the apex and the CEJ.7$8,l”p20Cores covered by a complete cast crown with a 2 mm margin on healthy tooth structure exhibited reduced stressconcentration at the apex and stress concentrated in the CEJ despite the presenceof the dowel or its designs The influence of core coverageby a complete cast crown with margins on healthy tooth structure wasexamined in similar studies evaluating the fracture resistanceof endo-
*References 5, 6,
11, 12, 17, 20, 22.
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T O FRACTURE
Cl
63.6%
c2
36.4% Fig.
1.
Fracture patterns in the various groups.
dontically treated teeth. There were significant differences between various dowel configurations and different core materials without a complete crown. Differences became insignificant after core coverage by a complete crown57 13*14*16*22,23 Conversely, differences were discovered between dowel design and core materials with teeth restored with complete cast crowns,6-12 and in one study, cast crowns were fabricated but not cemented.12 This study supported the assumption that post design did not influence the resistance to fracture of endodontitally treated teeth, provided the core was covered by a complete cast crown with a 2 mm margin on healthy tooth structure. The crown changed the distribution of forces to the root and the post and core complex, rendering the post characteristics insignificant. The forces responsible for failure were considerably higher than the maximal physiologic forces acting on the teeth in the oral cavity (Table I). A wide range in the average forces were discovered with in vitro studies of dowel-core systems restored with complete cast crowns.5’ 6*12-14*22*24However, there was some similarity between the magnitude of the forces causing failure in this research and in related studies.14p l6 Several factors contribute to the fracture resistance of teeth: (1) classification of the teeth (incisors versus molars); (2) degree of calcification; (3) distance from the CEJ at which loads are applied; (4) direction of force; (5) core height coronal to prepared tooth; and (6) the ferruling effect of the artificial crown margins. The rigidity of the material for embedding the tooth can also have an influence. A conservative approach of 1 mm in diameter was used in preparation of the root canal to preserve tooth structure. Preserving tooth structure during canal preparation is crucial to prevent stress concentration at the CEJ of the tooth21 and provide resistance to tooth fracture.g, 25 The dowel configuration is essential in a root if a thin dentinal wall remains after root canal preparation. This assumption
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Table
III.
Comparison of all possible group couples
Groups compared
P
Cl-C2 Cl-C3 Cl-C4 C2-C3 C2-C4 c3-c4
0.65 0.40 0.46 0.12 0.13 0.70
Statistical signi6cance
--_
--
NS NS NS NS NS NS
was not examined in this study, but is an additional factor that could explain the difference in results between this study and researchthat examined dowelsof larger diameter. Therefore, the remaining dentinal thicknesswassmaller 6,24-26
It was impossibleto compare the forces causingfailure in this study with similar investigation becauseof numerous variables. Standard deviations of 20% to 40% were observedin the median values (Table II) asa result of wide distribution in all groups (Table I). This could be attributed to (1) composition and thickness of the cementing agent, which was not standardized in all specimens;(2) dissimilar posit:ionsof the dowel in the prepared site; (3) variations in tooth dimensions;(4) minute variations in the morphology of root canals; and (5) variations in dentin causedby different water content, pulpal conditions before tooth extraction, patient’s age, and direction of dentinal channels. Similar standard deviations have been documented in the literature.16-1gx 23,24 The fracture lines in teeth containing a dowel-core USUally passedthrough the apex, but in some instances the fracture line passedthrough the apical third of the post (Fig. 1). The fracture line in the control group wassimil.ar in angle to other groups, but more coronal, and in the lingual surface or the surfaceopposite t,o the applied force, it 39
ASSIF
passed at the point where the tooth emerged from the acrylic resin cylinder. The position and angle of this fracture line were caused by the fulcrum in the lingual surface between the tooth embedded in the acrylic resin and the tooth surface coronal to the cylinder. The tooth then acted as a type 1 lever arm during force application on the facial surface. The fracture line passed more apically in the experimental groups containing a dowel; thus the dowel caused stress distribution along its length and shifted the stress concentration apically. In this study, there were no fractures parallel to the long axis of the root as in similar investigations when forces were applied directly to the core.23*24 Photoelastic studies disclosed that when forces were applied directly on tapered dowels, there was a wedging effect on the root?,sy lo However, if the core was covered by a complete crown, the wedging effect disappeared with a shift of stress concentration to the CElJ.8 This explained the longitudinal fractures in teeth with a dowel-core system without an artificial crown, but was not evident in teeth restored with a complete crown having 2 mm margins on healthy tooth structure. CONCLUSIONS The post design did not influence the fracture resistance of endodontically treated teeth having a dowel-core system of identical rigidity and restoration with a complete cast crown having a 2 mm margin on healthy tooth structure. The selection of a dowel should be based on a system that preserves the most tooth structure and possesses suitable retention of the core for restoration of the tooth. If the anatomic crown is sufficiently preserved and core retention can be achieved from within the natural crown, or if completion of the coronal surface is unnecessary, a dowel is not required. We express our deep appreciation to Dr. Tamar Brosh PhD (Engineering), Department of Oral Rehabilitation, for her invaluable assistance in performing the experiment; to Mrs. Ilana Gelerenter, Statistics Laboratory, for analysis of the data; to Whaledent International and their representatives in Israel for their assistance in acquiring equipment for the experiment; to Dentor Laboratories, Tel Aviv, for the casting of the post-cores and crowns; and to Ms. Rita Laxar for editorial assistance. REFERENCES Trabert KC, Coony JF’. The endodontically treated tooth. Restorative concepts and techniques. Dent Clin North Am 1984;28:923-51. 2. Colman HL. Restoration of endodonticslly treated teeth. Dent Clin 1.
North Am 1979;23:647-62. 3. Sokol DJ. Effective use of current DENT 1984,52:231-4.
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4. Baraban DJ. The restoration of endodontically treated teeth: an update. J PROSTHET DENT 198&59:553-S. 5. Hoag EP, Dwyer TG. A comparative evaluation of three post and core techniques. J PROSTHET DENT 1982;47:177-81. 6. Brandal JL, Nicholls JI, Harrington GW. A comparison of three restorative techniques for endodontically treated anterior teeth. J PROSTHET DENT
1987;58:161-5.
7. Coony JP, Caputo AA, Trabert KC. Retention and stress distribution of tapered-end endodontic posts. J PROSTHET DENT 1986;55:540-6. 8. Assif D, Oren E, Marshak BL, Aviv I. Photoelastic analysis of stress transfer by endodontically treated teeth to the supporting structure using different restorative techniques. J PROSTHET DENT 1989;61:535-43. 9. Standlee JP, Caputo AA, Holcomb J, Trabert KC. The retentive and stress distributing properties of a threaded endodontic dowel. J PROSTHET DENT
1980;44:398-404.
10. Standlee JP, Caputo AA, Pollack MM. Analysis of stress distribution by endodontic posts. Oral Surg, Oral Med, Oral Path01 1972;33:952-60. 11. Kantor ME, Pines MS. A comparative study of restorative techniques for pulpless teeth. J PROSTHET DENT 1977;38:405-12. 12. Chau RW, Bryant RW. Post-core foundations for endodontically treated teeth. J PROSTHET DENT 1982;48:401-6. 13. Gelfand M, Goldman M, Sunderman EJ. Effect of complete veneer crowns on the compressive strength of endodontically treated posterior teeth. J PROSTHET DENT 1984;52:635-8. 14. Nayyar A, McDonald TR, Turner F, Koth DL. Strength of premolar corona-radicular buildups restored with cast crowns [Abstract]. J Dent Res 1982;61:70. 15. Caputo AA, Standlee JP. Biomechanics in clinical dentistry. 1st ed. Chicago: Quintessence Publishing Co, 1987:134. 16. Michelich R, Nayyar A, Leonard L. Mechanical properties of amalgam core-buildups for endodontically treated premolars [Abstract]. J Dent Res 1981;60:1283. 17. Greenfeld RS, Roydhouse RH, Marshall FJ, Schoner B. A comparison of two post systems under applied compressive-shear loads. J PROSTHET DENT
1989;61:17-24.
18. Gusy GE, Nicholls JI. In vitro comparison of intact endodontically treated teeth with and without endo-post reinforcement. J PROSTHET DENT
1979;42:39-44.
19. Trope M, Malts DO, Tronstad L. Resistance to fracture of restored endodontically treated teeth. Endodon Dent Traumatol 1985;1:108-11. 20. Burns DA, Krause WR, Douglas HB, Burns DR. Stress distribution surrounding endodontic posts. J PROSTHET DENT 1990;64:412-8. 21. Hunter AJ, Feiglin B, Williams JF. Effects of post placement on endodontically treated teeth. J PROSTHET DENT 1989;62:166-72. 22. Volwiler RA, Nicholls JI, Harrington GW. A comparison of three core buildup materials used in conjunction with two post systems in endodontically treated anterior teeth. J Endodont 1989;15:355-61. 23. Barkhorder RA, Radke R, Abbasi J. Effect of metal collars on resistance of endodontically treated teeth to root fracture. J PROSTHET DENT 198%61:676-S.
24. Sorensen JA, Engelman MJ. Effect of post design on fracture resistance of endodontically treated teeth. J PROSTHET DENT 1990$X419-24. 25. Tjian AI-IL, Whaug SB. Resistance to root fracture of dowel channels with various thicknesses of buccal dentin walls. J PROSTHJZT DENT 1985;53:496-500.
26. Sorensen JA, Engelman MJ. Ferrule design and fracture resistance of endodontically treated teeth. J PROSTHET DENT 1990,63:529-36. Reprint
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DR. DAVID ASSIF SECTION OF ORAL REHABILITATION THE MAURICE AND GABRIELA GOLDSCHLECER TEL Av~v UNIVEFWTY TEL AVIV 69978 ISRAEL
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core and post concepts. J PROSTHET
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